The present invention relates generally to electrochemical systems, such as solid-oxide electrolyte fuel cells and fuel cell assemblies for directly converting chemical energy into electricity. More particularly, the present invention relates to a reversible fuel cell system having a plurality of functional modes, including a fuel cell mode, an electrolysis mode, and an electrical energy storage mode.
Planar, or flat, solid oxide fuel cell stacks are well known in the industry. Generally, a fuel cell is an electrochemical device which combines a fuel such as hydrogen with oxygen to produce electric power, heat and water. The solid oxide fuel cell consists of an anode, a cathode and an electrolyte. The anode and cathode are porous, thus allowing gases to pass through them. The electrolyte, located between the anode and cathode, is permeable only to oxygen ions as they pass from the cathode to the anode. The passing of the oxygen ions through the electrolyte creates an excess of electrons on the anode side to complete an electrical circuit through an external load to the cathode side, which is electron deficient.
A solid oxide fuel cell is very advantageous over conventional power generation systems. It is known in the industry that such devices are capable of delivering electric power with greater efficiency and lower emissions as compared to engine-generators.
Known planar solid oxide fuel cell stacks utilize a forced flow of gases through their electrodes. Furthermore, they employ fuel and air flow designs so that all, or at least many, of the cells are fed the same fuel and air compositions. The stacks are capable of producing good, but not optimal efficiencies. Furthermore, the stacks tend to exhibit significant local flow differences amongst cells and within cells. This can lead to increased stack performance degradation and a reduced stack efficiency. Further still, the stacks may require significant pressure drops, and therefore compression power, for the flowing gases.
Solid oxide electrolyzers are also known, which use input electric power to electrolyze steam into hydrogen and oxygen. Some types of known fuel cell stacks are capable of operation in either the fuel cell or electrolysis modes, while other technologies require separate stacks for fuel cell and electrolysis operation. A combination fuel cell/electrolyzer system can be used for electrical energy storage, using steam, hydrogen, and oxygen.
U.S. Pat. No. 4,770,955 (Ruhl) discloses a hollow planar solid oxide fuel cell employing forced fuel flow through each anode, with all anodes fed essentially the same fuel composition.
U.S. Pat. No. 5,198,310 (Fleming et al.) discloses a process for thermal management by feed gas conditioning in high temperature fuel cell systems wherein at least a portion of a fuel feed stream is chemically reacted in an exothermic chemical reaction in an external zone. The external zone is thermally separated from the fuel cell system and at least a portion of the products of the exothermic chemical reaction are passed to an internal zone in thermal exchange with the fuel cell system and reacted in an endothermic chemical reaction.
U.S. Pat. No. 5,340,664 (Hartvigsen) provides a thermally integrated heat exchange system for solid oxide electrolyte systems, which includes a thermally insulated furnace enclosure structure having an internal chamber therein and a plurality of solid oxide electrolyte plates disposed within the internal chamber.
U.S. Pat. No. 5,492,777 (Isenberg et al.) discloses an electrochemical energy conversion and storage system for storing electrical energy as chemical energy and recovering electrical energy from stored chemical energy. The solid oxide electrolyte electrochemical cell is operated in two modes: an energy storage mode and an energy recovery mode.
U.S. Pat. No. 5,733,675 (Dedrer et al.) discloses an electrochemical fuel cell generator having an internal and leak tight hydrocarbon fuel reformer.
European Patent No. 0 466 418 A1 (Ishihara et al.) discloses a solid oxide fuel cell and porous electrode for use with the fuel cell. The use of the porous electrode with the fuel cell improves the surface contact density at the interface while maintaining low resistance to gas diffusion in the electrode, enhancing power output.
A significant hurdle is that known combination fuel cell/electolyzer systems are capable of energy storage efficiencies no better than about 30%-40% energy in/energy out. Moreover, most such systems must use separate electrochemical stacks for fuel cell and electrolysis modes, thus adding to cost and complexity.
Thus, there is an unsatisfied need to have a reversible system using a single set of stacks for fuel cell and electrolysis modes and capable of achieving high fuel cell, electrolysis, and energy storage efficiencies.
The present invention is an electrochemical system being adapted to incorporate three different operating modes.
It is an object of the present invention to provide an electrochemical system having a fuel cell mode, and electrolysis mode, and an energy storage (reversible) mode.
It is another object of the present invention to provide an electrochemical system having an energy storage (reversible) mode that alternates between a fuel cell mode and an electrolysis mode, operating on hydrogen/steam mixtures and oxygen.
It is yet another object of the present invention to provide an electrochemical system that is a reversible system using a single set of stacks, or a plurality of stacks, for fuel cell and electrolysis modes and capable of achieving high fuel cell, electrolysis, and energy storage efficiencies.
The system of the present invention is designed to be capable of being used in several various modes of operation. The first is a fuel/air fuel cell mode wherein power is generated. Propane is used as an example of a fuel which contains both hydrogen and carbon, but similar operation can be achieved with other hydrocarbons and with oxygenates, including methanol, ethanol, biogas, gasifier gas and landfill gas, provided that certain impurities such as particulates and metals are removed.
In this mode, syngas, which consist mainly of hydrogen, water, carbon monoxide and carbon dioxide, is produced in a reformer from propane and steam and heated using surplus heat from the stack(s). Hot syngas, also called hot fuel gas, is fed to a fuel manifold at a controlled flow rate. Hot air is fed to a hot air manifold at a controlled flow rate. When the external electrical circuit is closed, the cell stack generates electric power from the electrochemical combination of fuel and oxygen molecules. Oxygen from the air diffuses inwards in each oxygen electrode, via gaseous diffusion. Fuel molecules, hydrogen and carbon monoxide, diffuse outwards and water and carbon dioxide molecules diffuse inwards within the fuel cell electrode, also via gaseous diffusion. Secondary non-electrochemical reactions also occur both in the fuel electrode and in the fuel manifold. These include the water-gas shift reaction represented by the equation:
CO+H2O(g)xe2x86x92CO2+H2
and the steam reforming of residual hydrocarbons represented by the equation:
CH4+H2O(g)xe2x86x92CO2+3H2.
As the syngas and air flow through their manifolds past the cell stack, the fuel becomes progressively oxidized and the air becomes progressively depleted of oxygen. The cell operating voltages will vary along the stack based upon the chemical potential of the local fuel and oxidizer compositions. This allows high total stack voltage and hence high stack efficiencies. By selecting stack operating conditions properly, electrochemical fuel utilizations up to 100% are possible, especially when using oxygen-tolerant fuel electrodes at the fuel exit end of the stack. High fuel utilization also boosts efficiencies.
During fuel/air operation, incoming air is partially preheated and serves to remove excess heat from the stack, thereby allowing control of cell stack operating temperature for an unlimited duration.
A second mode is an electrolysis mode. In this mode, liquid water is vaporized and then electrolyzed using an electric power input to produce hydrogen and oxygen for storage or other uses.
A third mode is the alternating operation of electrolysis mode with a hydrogen/oxygen fuel cell mode to comprise a highly efficient electrical energy storage system. A hot thermal mass is used to absorb excess heat produced in the fuel cell mode for later release during the electrolysis mode, via a temperature swing of the hot thermal mass. Another thermal mass absorbs excess heat from spent fuel cooling in fuel cell mode for later use in electrolysis mode to help preheat the feeds. Electric heaters are employed as needed to prevent the thermal mass temperature from falling below a preset limit during extended periods of low-power electrolysis.
The system is also capable of operation on various other fuel/oxidizer combinations.
The multipurpose reversible electrochemical system of the present invention provides various advantages over fuel cells and electrolyzers of the prior art. For example, the cell stacks of the present invention do not require close matching of electrodes, nor having to tailor them for low forced-gas pressure drops. Systems may be operated with low fuel and air supply pressures, thus minimizing costs and energy consumption for blowers, etc.